Carl Rollyson must be awfully keen on Martha Gellhorn, the exceptional correspondent from the 20th century's troubled zones and, alas, indifferent novelist. Because Beautiful Exile: The Life of Martha Gellhorn, is his second attempt at a biography of her.

His first, Nothing Ever Happens To The Brave, published in 1990 when she was still alive, was not much noticed, except by Gellhorn, who read it "with horrid care" and rightly complained that Rollyson had "transposed her fiction directly into her life with weird effects". Nothing was not a top-quality job, but Rollyson did what the book's subtitle - The Story of Martha Gellhorn - specified and narrated a tale not then very well-known. There were those inept transpositions about which Gellhorn complained, but there were also warranted facts: for example, that Gellhorn had later admitted she had not been present at a southern lynching of which she had written such a vivid eyewitness account, still in print.

Exasperatingly, Rollyson failed to address that incident further, although it may well have been the source of Gellhorn's ever thereafter furious pursuit of truth, exactitude about detail, and contempt for "apocryphiars", as she damned those who bullshat and falsified journalism's first draft of history.

You are about to ask: why go on about Rollyson's first Gellhorn tome? Because he does not. It is absent from the new book's PR and jacket credits. He mentions only briefly in its introduction that, while biographing Rebecca West: A Saga of the Century, (he specialises in "iconic" females, like Lillian Hellman and Susan Sontag), he "began to collect material" for a Gellhorn volume of "increased scope". The scope that has been increased (dilated would be a more accurate verb) is, of course, mostly sexual.

The reference to the lynching piece, which Gellhorn dashed off while staying in the London house of HG Wells, has been cleared away, presumably to make space for Rollyson's speculation based on an unsubstantiated passage in a suppressed section of the third volume of Wells's autobiography - now there's a relay of tenuousness - that Wells, nearly 70, and Gellhorn, 27, were lovers.

Wells wrote that they made love; he would have "a time with her when next I go to America", although she was "extremely incidental". Gellhorn rebutted his carnal assertions retrospectively: why would she sleep with "a little old man when I could have any number of tall beautiful young men"?

The effect of such diminished information - Beautiful is over 100 pages shorter than Nothing - as well as of changed emphasis, is to make the second book rather worse than the first. Rollyson remains ungrudging about Gellhorn's gifts, praising what Wells described as the "instinctive directness and vigour" of her reports, and he is spot-on about the profundity of her "miniatures of war", even if he does annoyingly precis their content without matching her terse syntax. But he does not really seem engaged by the construction of her work, her inner life, or her milieu and times.

His stated intention is to answer the question he believes Gellhorn's devotees and obituarists did not ask: how she got to be in the right place at the right time, in a doorway in a Madrid street under Spanish civil war bombardment, flat on her face on Omaha beach in Normandy on D-Day plus one, or snowed-in in the Ardennes during the Battle of the Bulge. He concludes that she used her beauty - she was blonde with a glowing pure skin, and her stylishness (she sashayed in Saks Fifth Avenue slacks on the front line) - to seduce and manipulate men, though not always physically; that she invented and controlled her public image with miserly tightness; that she was a careerist to the end, finally charming a whole new generation of literary chaps into perpetuating her legend.

In his disappointment with the failings of her character, Rollyson comes over as far more naive than the friends and obituarists, most of whom are journos and therefore hold it to be self-evident that it takes a woman more than just a proven talent for prose to wangle a commission up the sharp end and get her dispatches in print. (Although it may not involve sleeping with men, more going along with their fantasies of sexy girls combat-kitted in vile places.)

Poor Rollyson. He rakes the wrong muck. And he is too morally didactic to enjoy, as a biographer must, the complexities and ambiguities of his subject. Consider the love stuff, since it is the main addition. He rejects Gellhorn's evaluations of her relationships with her first two husbands, the journalist Bertrand de Jouvenel and a certain Ernest Hemingway, exceptional novelist and indifferent correspondent, although he relishes rehashing the legendary first barstool encounter of Hem and Marty, the pursuits and disillusionments, and the campaign of mutual post-marital hate conducted among the ruins.

But since Rollyson has a thesis this time, Gellhorn as designing woman, he is constantly incurious about fascinating details turned up by his research that do not serve the agenda. The most riveting aspect of Hem and M is not Hem's claims that M had a vaginoplasty, the better to admit him, or that she was an aspirant bitch hefting a vanity case in her pack, but how much of the romance on both sides seems to have been based on trashy fantasy. She admired his novels as a student long before she knew him, trying to live as an EH heroine: while he, in letters to her, cast both of them as leads in a fan-fiction version of his own novels ("we have been pursued by and fought gangsters, the Gestapo, Nazi troops etc the type where you hold them and I squash their heads in with a rock"). So Hem wrote notes calling her "Dearest Pocklechuck" and saying how much "Mr Scrooby" - his pet name for his penis - needed her. And they are in an archive. The father of modern American punchy prose, on public record sounding like a scriptwriter for Men Behaving Badly - now that's far more interesting than any correspondence Gellhorn might have burnt before her death.

Oddly, Rollyson does accept, without much question, Gellhorn's old-age reminiscences of the sexual incidents of her wartime life, her explanation that her one-night stands with soldiery grew out of friendship and compassion, a treat offered to men who wanted sex very much, though the act itself left her cold. So she was false and coy about the husbands but honest about the fucks, eh? Perhaps.

Irritatingly, he barely outlines her affair with General James Gavin of the 82nd Airborne, who, by the Rollyson definition, was her access to all areas of the Bulge fighting. For Rollyson, Gavin demonstrates chiefly the Gellhorn weakness for hero-figures; so he omits to fill in much about the romance, or the identity of the 23-year-old Gavin married post-war, or what Gellhorn felt about losing a partner she genuinely respected, or just how she influenced Gavin's world view and remained his friend for life. We would really have liked to know more.

The understanding of life's improvisations and limitations essential to biography have gone missing in action in Beautiful Exile. Now if Victoria Glendinning, whom Rollyson occasionally quotes, had taken on the assignment, it would have been different: we could have heard Gellhorn bashing the portable typewriter in wrath in the room upstairs.